Environmental niche overlap in sibling planktonic species Calanus finmarchicus and C. glacialis in Arctic fjords

Abstract Knowledge of environmental preferences of the key planktonic species, such as Calanus copepods in the Arctic, is crucial to understand ecosystem function and its future under climate change. Here, we assessed the environmental conditions influencing the development stages of Atlantic Calanus finmarchicus and Arctic Calanus glacialis, and we quantified the extent to which their niches overlap by incorporating multiple environmental data. We based our analysis on a 3‐year seasonal collection of zooplankton by sediment traps, located on moorings in two contrasting Svalbard fjords: the Arctic Rijpfjorden and the Atlantic‐influenced Kongsfjorden. Despite large differences in water temperature between the fjords, local realized ecological niches of the sibling Calanus species overlapped almost perfectly. The exception was the earliest copepodites of C. glacialis in Rijpfjorden, which probably utilized the local ice algal bloom in spring. However, during periods with no sea ice, like in Kongsfjorden, the siblings of both Calanus species showed high synchronization in the population structure. Interestingly, differences in temperature preferences of C. finmarchicus and C. glacialis were much higher between the studied fjords than between the species. Our analysis confirmed the high plasticity of Calanus copepods and their abilities to adapt to highly variable environmental settings, not only on an interannual basis but also in a climate warming context, indicating some resilience in the Calanus community.

finmarchicus and Calanus glacialis are among the main contributors to the zooplankton biomass in Arctic shelf seas (Aarflot et al., 2018).
Calanus glacialis is an Arctic endemic species with a panmictic population with large-scale gene flow across the peripheral seas of the Arctic Ocean (Weydmann et al., 2016). The boreal C. finmarchicus has its center of distribution in the large gyres of the North Atlantic, but it is regularly transported with ocean currents of Atlantic origin to the Arctic, where it can reach high biomass (Aarflot et al., 2018;Carstensen et al., 2012;Kosobokova & Hirche, 2009). The life cycle of both species includes an overwintering phase at depth, while their reproduction, growth, and development are closely coupled to the short but intense algal blooms in surface waters during spring and summer. During the summer, they build up their large lipid reserves, which sustain them through the winter and fuel molting and maturation in spring. Differences in life history strategies between the two species reflect adaptations to the environmental settings in their core areas of distribution .
Calanus glacialis is adapted to the environmental conditions of seasonal ice-covered seas, where low temperatures prevail and the onset and duration of the phytoplankton spring bloom can vary substantially between years, but which may also provide an additional food source early in spring in the form of an ice algal bloom (Daase et al., 2013;Søreide et al., 2010). Reproduction can be fueled on lipid reserves alone, thus enabling C. glacialis to initiate egg production prior to the onset of the spring bloom, but it will also utilize the ice algal bloom to increase the reproductive output. Under favorable conditions, reproduction is timed in a way that allows young developmental stages to fully utilize the phytoplankton spring bloom (Søreide et al., 2010). As the season of high food availability can be short and growth is slow at low temperatures (Weydmann et al., 2015), C. glacialis needs 1-2 years to fulfill its life cycle (Daase et al., 2013). The option to extend its life cycle over 2 years gives this Arctic copepod the flexibility to cope with the high environmental variability in seasonally ice-covered seas, and allows it to grow larger and accumulate larger lipid reserves than the smaller, less lipid-rich Atlantic C. finmarchicus.
Calanus finmarchicus is adapted to a boreal environment, where the timing of the spring bloom is more predictable, the primary production season is longer and where warmer temperatures ensure faster development, making it possible to complete the life cycle in 1 year (at the northern extend of its distributional ranges) or even less (multigenerational life cycle in the southern end of its distribution; Weydmann et al., 2018). This comes at the cost of smaller body size and smaller lipid reserves compared to the Arctic Calanus species, and C. finmarchicus therefore has to rely on external food supply to fuel reproduction in spring as its lipid reserves are not sufficient. While C. finmarchicus is regularly transported into the Arctic, the short pelagic algae growing season and low temperatures limit its ability to reproduce and to reach older developmental stages with sufficient lipid reserves that would allow successful overwintering in the Arctic (Hirche & Kosobokova, 2007;Ji et al., 2012). However, with climate warming leading to less sea ice, increased water temperatures, and longer algae growth season, there are indications that conditions may become more favorable for C. finmarchicus in the Arctic. A poleward shift and an increase of C. finmarchicus contribution to the overall Calanus biomass has been already observed in some Arctic regions (Carstensen et al., 2012;Chust et al., 2014;Hop, Assmy, et al., 2019;Møller & Nielsen, 2020;Weydmann et al., 2014), and C. finmarchicus may be able to accelerate its development leading to potentially produce a second generation in warmer years (Weydmann et al., 2018).
Despite these differences in life history strategies, spatial distribution of C. finmarchicus and C. glacialis largely overlaps in the European Arctic, especially in the areas that are regularly affected by the influx of Atlantic water masses (Aarflot et al., 2018;Carstensen et al., 2012;Walkusz et al., 2009;Weydmann et al., 2013;Willis et al., 2006). However, due to harsh winter conditions and sea ice coverage, studies on the sibling Calanus species and other important zooplankton taxa in the European Arctic are usually limited to late spring-early autumn that bias the understanding of their biology and environmental preferences. Further, studies are often based on different sampling approaches than traditional net collection (Weydmann-Zwolicka, Balazy, et al., 2021), such as the use of sediment traps that allow for the year-round continuous zooplankton collection (Weydmann-Zwolicka, Prątnicka, et al., 2021;Willis et al., 2008). Both species are known to exhibit high plasticity in relation to environmental conditions (Trudnowska et al., 2020), although routinely measured hydrographic properties of water masses, such as temperature and salinity, seem to be the key in predicting their distribution in the European Arctic: C. finmarchicus is generally found in warmer and more saline waters of Atlantic origin, whereas C. glacialis is associated with colder, Arctic waters (Aarflot et al., 2018;Daase et al., 2007;Hop et al., 2002;Weydmann et al., 2013;Weydmann & Kwaśniewski, 2008). However, in some Svalbard fjords, main difference between C. glacialis and C. finmarchicus life traits was based on different timing in reproduction, what reduced species competition, although they were reported to occupy similar environmental niches (Hatlebakk et al., 2022).
Environmental niche corresponds to abiotic factors which, according to the classical definition of an ecological niche describing it as a multidimensional hypervolume in which a species maintains a viable population, influence birth and death rates together with biotic factors (Hutchinson, 1957). Species niches overlap when cooccurring species share parts of their niche spaces with each other and, consequently, compete for resources. The degree to which species niches overlap may give insight into their interactions, e.g., high niche overlap indicates competition for resources, and may lead to exclusion for some species; while low niche overlap generally implies utilization of different resources, and thus lower levels of species interactions (Tsafack et al., 2021). Such interactions are especially important for species inhabiting regions that are exposed to rapid environmental changes, such as the recent Arctic.
It is impossible to measure all variables responsible for shared environmental niches of marine zooplankton species, thus any statistical attempts must be based on limited, and possibly routinely measured, environmental data (Beaugrand & Helaouët, 2008;Freer et al., 2022;Hatlebakk et al., 2022). The major objective of our study was to assess C. finmarchicus and C. glacialis environmental preferences, and the degree to which environmental niches of their development stages overlap, with the use of data on temperature, salinity, and chlorophyll a fluorescence, obtained from moorings located in two contrasting Svalbard fjords: the high-Arctic Rijpfjorden and Atlantic-influenced Kongsfjorden, and coupled with the 3-year seasonal collection of zooplankton by the attached sediment traps.
Based on the results, we also tried to predict the future of Calanus complex in a climate warming context.

| Study area
Two contrasting fjords in Svalbard, a high-Arctic archipelago, were selected as the study areas: Kongsfjorden and Rijpfjorden ( Figure 1). Kongsfjorden, a west-facing fjord of Spitsbergen, is regularly affected by influx of Atlantic water masses advected from the West Spitsbergen Current over the adjacent shelf area (Cottier et al., 2005;Promińska et al., 2017). A long-term time series of temperature measurements in the fjord indicate that the water temperature has increased in the order of 1°C per decade since 2002 (Cottier et al., 2019). Consequently, Kongsfjorden has been mostly ice-free since winter -2006(Cottier et al., 2007Walczowski et al., 2012). The hydrographic settings in Kongsfjorden strongly influence the zooplankton community structure, which shows high seasonal, spatial, and interannual variation, with high abundances of Atlantic species (Hop et al., 2002;Walkusz et al., 2009;Weydmann-Zwolicka, Prątnicka, et al., 2021;Willis et al., 2006Willis et al., , 2008. Rijpfjorden is a north-facing fjord located on Nordaustlandet (80°N), it is predominantly ice-covered for at least 6-8 months of the year and experiences oceanographic conditions that are more Arctic in character (relatively cold and fresh). Its zooplankton community is dominated by Arctic species, although species of boreal and Atlantic origin are also present, especially when waters of Atlantic origin are advected into the fjord (Hop, Assmy, et al., 2019;Weydmann et al., 2013;Weydmann-Zwolicka, Prątnicka, et al., 2021). No significant increase in temperature has been observed in Rijpfjorden since long-term monitoring began in 2006 (Cottier et al., 2019). provide a density discontinuity relative to ambient seawater and to avoid diffusion of samples. To preserve deposited material during and after deployment, 4% formalin buffered with sodium borate was added to each sample cup. Temperature (T) and salinity (S) were recorded during the sediment trap deployment period with Seabird 37

| Sampling and laboratory work
Microcats moored at a depth of 60 m. Additional Seapoint Sensors were mounted at 25 m and used to measure chlorophyll a fluorescence (Chl-a), and the presence or absence of sea ice was interfered from data collected by upwards looking Acoustic Doppler Current Profilers (ADCP; Hyatt et al., 2008).
Detailed laboratory procedures of handling the sediment trap samples are described in Weydmann-Zwolicka, Prątnicka, et al. (2021). Calanus individuals were identified to species based on their morphology (Brodskii et al., 1983), and prosome lengths (distance from the apex of head to the distal margin of the last thoracic segment, laterally) of individual copepodite stages (C1-C5, and adult females and males) according to Weydmann and Kwaśniewski (2008) under a stereomicroscope equipped with a calibrated ocular micrometer (Leica M125 C; Leica Imaging Systems GmbH). Due to relatively small differences in the prosome length of earlier copepodites, C1-C3 were measured at the magnification of at least × 25. Only individuals that showed no signs of decomposition, suggesting they were killed by the applied formaldehyde, were chosen for further analyses (Matsuno et al., 2014).

| Statistical analyses
Due to possible disturbances connected with sediment trap deployment, the first samples from each cycle (24.08-01.10) were excluded from further analyses, resulting in 20 samples collected during each year in each fjord, and a total of 120 samples. After excluding the samples in which neither C. finmarchicus nor C. glacialis was present, we obtained 100 samples, which were further analyzed. Sediment trap exposure time varied between the seasons (Weydmann-Zwolicka, Prątnicka, et al., 2021); hence, we standardized Calanus abundance data to a daily flux, expressed as individual m −2 day −1 , according to Matsuno et al. (2014), and these data were used in the following analyses. Because the bloom intensity differed between the years and fjords, the raw Chl-a fluorescence values were recalculated in the range of 0-100 for each year/fjord. This way the differences in Chl-a fluorescence values between the fjords and years did not mask the bloom timing and duration and, thus its influence on Calanus species phenology. For the purpose of statistical analyses, temperature, salinity, and fluorescence data were aver- PERMANOVA was run under the following conditions: type III (partial) sums of squares, permutation of residuals under a reduced model, 999 permutations, and fixed effects sum to zero for mixed terms.
The influence of the continuous environmental variables, averaged water temperature, salinity, day length, and Chl-a fluorescence on the studied Calanus copepodites daily flux, was tested with a distancebased linear model (DistLM) routine, illustrated by distance-based redundancy analysis (dbRDA) diagrams. The Calanus daily flux data were transformed [n′ = log (n + 1)] prior to further analyses, and similarities between samples were examined using the Bray-Curtis index.
To build the models, all specified predictor variables were included using a forward selection procedure, and the selection criterion was based on adjusted R 2 values. Because predictor variables jointly affect species composition in marine environments, we showed only the results of sequential tests (Legendre & Anderson, 1999). Both types of analyses were performed using PRIMER 7 software package (Clarke & Warwick, 2001), with the PERMANOVA+ add on (Anderson et al., 2008). This approach allowed to provide quantitative measures of variation in Calanus daily flux explained by both types of predictor variables: factors and continuous variables, separately, so the analysis was not biased by possible correlations between some variables, like temperature and months.
Niche-based models that are calibrated in the native range by relating species observations to climatic variables are commonly used to predict their future spatial extent (Broennimann et al., 2007).
Statistical procedures, which were used to quantify and compare ecological niches of the North Atlantic Calanus copepods, have been proposed for C. finmarchicus and C. helgolandicus and were based on three environmental factors: temperature, salinity, and bathymetry (Beaugrand & Helaouët, 2008). Since the degree of C. finmarchicus and C. glacialis ecological niche overlap in the high-Arctic fjords, in which these sibling species co-occur, has never been quantified, we decided to calculate the average niche overlap (NO) values between the pairs of studied species and their development stages, also by incorporating three routinely measured environmental predictors: averaged water temperature and salinity, as well as standardized Chl-a fluorescence data, which were selected based on the results of DistLM, and were analyzed separately for each fjord, Rijpfjorden and Kongsfjorden.
To calculate environmental niche overlap for C. finmarchicus and C. glacialis, we used the unified analysis of NO described in Geange et al. (2011) and the R codes provided in this publication. For each suite of analyses, we used appropriate transformations and probability models (as described in Geange et al., 2011) to measure niche overlap between the pairs of development stages and over each different niche axis, here understood as temperature, salinity, and Chl-a fluorescence. Measurements derived from different environmental data were then combined into a single, unified analysis of NO by averaging over multiple axes, and null model permutation tests were used to assess statistical differences in niche overlap. This procedure led to directly comparable measures of NO, with the overlap statistic between two sibling Calanus copepods defined as the overlapping area between the distributions for each species; where NO ranged between 0 (two distributions are completely separated) and 1 (distributions exactly coincide). The obtained matrix of calculated NO dissimilarities (1-NO) of Calanus development stages was then used to graphically display the results for each fjord with the use of nonmetric multidimensional scaling (nMDS), and the Spearman rank correlation between the matching resemblance matrices was tested by RELATE routine in PRIMER 7.
Afterwards, two-way ANOVA was performed in R (RStudio Team, 2015;R CoreTeam, 2017) to test if the niche position across temperature axis, which was the most influential environmental variable according to DistLM and the most differentiating single niche axis, significantly differed between C. finmarchicus and C. glacialis and between Rijpfjorden and Kongsfjorden.

| Environmental factors and variables influencing Calanus
The daily flux of certain development stages of Calanus copepods differed between the fjords and seasons (Figure 2), although it is worth noticing that the population structure in both fjords was similar during each autumn, when the latest copepodites (C4-C5) dominated, and in winters, when the increasing proportion of adults was observed. Then, in the following seasons, the earlier copepodites (C1-C2) appeared in Kongsfjorden around May, while in Rijpfjorden, they were noted around July-August, after the breakup of the sea ice. Such a situation was observed in the two sampling According to PERMANOVA, two tested single factors significantly affected the daily flux of Calanus development stages, e.g., calendar months, which explained 23.9% of variation in C. finmarchicus and C. glacialis development stages daily flux, and sampling years with nested months, responsible for 25% (Table 1). The fjord alone was not among the significant factors; however, through the interaction with the other factors, it significantly influenced the daily flux of the studied Calanus species.
All tested continuous environmental variables significantly affected the daily flux of the studied Calanus development stages, although they explained reasonably low proportion of variance. The most influential variable was temperature, which was responsible for 5.39% variation in data, followed by daylength 5.09%, salinity 3.13%, and fluorescence 1.92% (Table 2). The importance of the first variable is well illustrated in the dbRDA ordination plot (Figure 3

| Environmental niche overlap
In Kongsfjorden, the environmental niches of C. finmarchicus males and earlier copepodite stages were significantly different and most dissimilar of all the developmental stages of both Calanus copepods (Table 3; Figure 4). According to the results for each axis/environmental variable separately, which are presented in the Supporting Information, the main differences between development stages in this Atlantic-influenced fjord were also connected to salinity preferences, in which C. finmarchicus males differed significantly from the remaining copepodites of the same species (apart from C2), as well as from C. glacialis C4 and C5; while C. glacialis females differed from C1, C3, C4, C5 and females of C. finmarchicus, as well as from C4 and C5 of C. glacialis (Table S1a). Additionally, the males of the Atlantic species differed from its C3, C5 and females in terms of Chl-a fluorescence preferences (Table S1c). Only two significant differences in temperature distributions were observed in Kongsfjorden: between C. glacialis C3 and its females and C5 of C.
finmarchicus (Table S1b). Accordingly, in the nMDS plot (Figure 4), two main groups can be seen: (1) C4, C5, and females; and (2) C1-C3 of both species, with males being outliers. It is worth noting that fjords C4, C5 and females of the sibling Calanus copepods are clustered more closely than the other development stages, probably due to their high numbers in the autumn in both fjords.

F I G U R E 2
The flux of Calanus finmarchicus and C. glacialis development stages (copepodites C1-C5, females, and males), temperatures at a depth of 60 m (black line), and standardized fluorescence, averaged for the respective sediment traps exposition time, between October 2015 and August 2017. The presence of sea-ice coverage in Rijpfjorden is shown as blue bars.  Figure 5).

| DISCUSS ION
In both investigated Svalbard fjords, the local realized environmental niches of C. finmarchicus and C. glacialis overlapped almost perfectly.
Large similarities in population structure confirmed the synchronized development of C. finmarchicus and C. glacialis populations in high Arctic fjords, despite large differences in environmental conditions between the studied fjords: Arctic Rijpfjorden and Atlanticinfluenced Kongsfjorden. The main difference in the phenology of these sibling Calanus species between the fjords was linked to the presence of sea ice, which caused a 2-to 3-month shift in age population structure, with an earlier start in the warmer and ice-free Kongsfjorden compared to Rijpfjorden. A similar delay in zooplankton phenology between these fjords has been previously shown by Weydmann et al. (2013) and Weydmann-Zwolicka, Prątnicka, et al. (2021), and was most likely connected to the onset of phytoplankton bloom, which was exploited by both species.
Although it is difficult to disentangle between Calanus species phenology and highly seasonal annual cycle in the high Arctic, what means that environmental conditions, and thus environmental niche, changes seasonally; it is striking that differences in temperature preferences of the sibling copepods were much higher between the studied fjords than between the species.
Therefore, both sibling Calanus species exhibited similar phenol-  Note: Significant predictors are given in bold.

F I G U R E 3
Distance-based RDA plot illustrating the ordination of samples from Rijpfjorden (circles) and Kongsfjorden (squares), with environmental variables and Calanus (Cf, C. finmarchicus and Cg, C. glacialis) development stages (C1-C5; males, M; and females, F).
However, none of these studies quantified species niche overlap, and they were based on different data and statistical analyses, therefore likely methodological approach was responsible for the observed differences. The lack of environmental niche differentiation between the sibling Calanus species was also observed during year-round study from Isfjorden, a fjord on the west coast of Spitsbergen, suggesting that both species may benefit from warming due to accelerated growth and higher survival of the recruits (Hatlebakk et al., 2022). The presented realized niches for C.
finmarchicus and C. glacialis suggest high plasticity in both species.
This is in agreement with Trudnowska et al. (2020) who reported high plasticity and synchronization in the population age structure between the two Calanus species in Hornsund fjord, and the adjacent southern part of the West Spitsbergen shelf.
Based on the similarities between the local realized niches, the main groups consisting of the latest copepodites (C4-C5 and fe-

males) of both Calanus species could be distinguished in both fjords
in autumn-winter. Such large niche overlap, especially connected to temperature axis, and the correlation of the later copepodite stages to this variable revealed by dbRDA were connected with the fact that in autumn, when water temperatures in the fjords were the warmest, later copepodites were preparing to overwinter at greater depths . The distinguishable grouping of the earlier development stages in Kongsfjorden, observed mainly in spring, and the lack of such a grouping in Rijpfjorden were probably connected with food availability. Such variability in C. glacialis reproduction was reported by Daase et al. (2013), who noticed that in TA B L E 3 Mean niche overlap (upper) and its probability identified by the null model tests (lower) between the pairs of Calanus finmarchicus (Cf) and C. glacialis (Cg) development stages (copepodites, C1-C5; females, F; and males, M) in Kongsfjorden.

F I G U R E 4
Similarities in ecological niche overlap between Calanus finmarchicus (red copepods) and C. glacialis (dark blue copepods) development stages (C1-C5; males, M; and females, F) in Svalbard fjords, represented graphically as nonmetric multidimensional scaling.
Rijpfjorden the Arctic copepod utilized the ice algae bloom to fuel spawning in spring. Conversely, in Kongsfjorden, C. glacialis was spawning earlier in the season, even in the absence of food, what allowed to support growth and development of the new generation by the phytoplankton bloom. Unfortunately, there were no reliable data on feeding conditions that could have been included to our analyses, although the measurements of chlorophyll a fluorescence that were taken during sediment traps exposure may still help to understand processes taking place in Svalbard fjords.
Importantly, it should be mentioned that the observed pattern of the prevailing flux of the later copepodites and the scarce presence of the earlier ones in the sediment traps installed at 60 m depth might be biased, due to distinct biology, mobility capabilities, and the noted depth preferences of Calanus development stages. Especially, data on the earliest copepodite stages, which usually feed on the blooms in the surface layers (Søreide et al., 2010), should be treated with caution, given the sediment traps' deployment depth and difficulties in distinguishing the earliest copepodite stages of C. finmarchicus and C. glacialis based on prosome length. Interpreting results only based on qualitative data at one specific depth, treated as a proxy of the position of the organisms in the water column, is one of the limitations of using sediment traps to collect zooplankton (Dezutter et al., 2019). However, the continuous collection of zooplankton and environmental data still seems to be a more complete method to quantify niche overlap between planktonic species than traditional single net tows. In fact, the use of automatic methods that allow for a continuous collection of zooplankton and environmental data is often the only available option in the high Arctic TA B L E 4 Mean niche overlap (upper) and its probability identified by null model tests (lower) between the pairs of Calanus finmarchicus (Cf) and C. glacialis (Cg) development stages (copepodites, C1-C5; females, F; and males, M) in Rijpfjorden.  (Karnovsky et al., 2003;Stempniewicz et al., 2021). And under the perception that energy content is solely species dependent, a change in the species composition can potentially have severe effects on energy transfer to higher trophic levels. With climate warming leading to less sea ice, increased water temperatures and longer algae growth season, there are indications that C. finmarchicus may become more successful in establishing itself in the Arctic, and a poleward shift and an increasing C. finmarchicus contribution to the overall Calanus biomass has already been observed in some Arctic regions (Aarflot et al., 2018;Chust et al., 2014;Møller & Nielsen, 2020;Weydmann et al., 2014), thus changing prey size field and energy content for Calanus predators. However, so far there is no evidence that C. glacialis abundance or biomass is decreasing Møller & Nielsen, 2020). Furthermore, a trait-based model suggested that climate-driven changes in bloom phenology and temperature may drive the Arctic C. glacialis to adapt life history strategies similar to those of C. finmarchicus, i.e., shorter generation time and smaller body size (Renaud et al., 2018).
Thus, under climate warming scenarios, we can expect that the Calanus community in the Arctic will change toward populations consisting of individuals with smaller body size and less lipid content.
But this may not be due to a change in species composition but due to changes in life history strategies in the co-existing species. The large overlap in environmental niches between C. finmarchicus and C. glacialis observed in the present study suggests that both species may be able to adapt to highly variable environmental settings, not only on an interannual basis but also in a climate warming context, indicating some resilience in the Calanus community.

DATA AVA I L A B I L I T Y S TAT E M E N T
The detailed results of niche overlap analyses, as supplementary ta-